One year one - regression

Hi, We had a struggle to train DS1, took a while, and finally got there a few months past 3. A year later, and this last week, we've had 3 soiled pants, and numerous wet trousers.Any ideas where to go? I lost it tonight, after he'd tried to clean up after himself - ie poo EVERYWHERE - walls, clothes, floors, toilet seat you name it. He's been threatened with nappies if he doesn't improve - probably not the best route, so what should I be doing??